In conventional seismic exploration, field operations are preceded by the preparation of seismic plans in which lines are drawn on a map to indicate where seismic experiments will be conducted. Such seismic plans also set forth such other requirements as: the field configurations (geometrical patterns) of geophones desired for each seismic experiment, the number and sequence of all the seismic experiments to be conducted, the quantity of data to be collected, and the manner in which the data will be collected. The plan is prepared to insure the effective collection of data which will yield the most useful information concerning the underlying formations of a particular geographic area.
As with any scientific experiment, the quantity and quality of data obtained directly affects the ability to predict results from the data. In seismic exploration, the quantity and quality of data directly affects the ability of the geophysicist to make predictions about the subsurface geology. One way to improve the quality of data is to make multiple observations of the same subsurface reflecting points within a formation. Multiple observations allow better interpretation of data and, through the use of known "noise reduction" techniques, multiple observations also allow the elimination of undesired signal components during subsequent data processing.
Field methods for obtaining multiple data on common reflecting points are generally referred to as "stacking" or "Common Depth Point" surveying methods. Stacking methods include the performance of numerous seismic experiments utilizing varying geophone or geophone-array geometrical configurations. Stacking methods also involve switching the data gathering operation from one active geophone to a group of active geophones.
It is well known that energy traveling in the horizontal plane away from a seismic energy source arrives at different geophone groups at different times. The first arrival occurs at the geophone group nearest the source, and then progresses to those increasingly further away from the source. The result is that even though groups of geophones are typically connected electrically by wires so that data is recorded as though there exists only one geophone, horizontally traveling energy (which generally provides little information about deep formations) is recorded out of phase, and some signals cancel other signals. Energy traveling in the vertical plane, on the other hand, arrives at the geophones at about the same time, or more or less in phase, so signals tend to reinforce each other. Vertically traveling energy, which provides the most useful information concerning the underlying regions, is thus more easily perceived in the data record sections. These concepts, combined with advances in seismic instrumentation, provide an opportunity during data gathering and data processing to cancel out unwanted signals detected at the geophones. Data recording is thus typically effected through geophones and arrays of geophones arranged in the field so as to take advantage of well established "noise" cancellation techniques. Noise cancellation techniques and methods also involve arranging the geophones in the field so as to cancel specific types of noise for a specific situation. For example, geophones in a group may be unevenly spaced or "tapered" in order to specifically reduce the type of noise encountered in a particular area. The arrangement and rearrangement of geophones in the field involve numerous connections and disconnections of geophones, groups of geophones and arrays of geophones. Field control of these connection and disconnection operations is typically effected by connecting all geophones in the field to the recording equipment via an electrical cable.
The cables utilized in seismic data gathering operations are typically very complex cables that contain numerous instruments along their lengths. These cables are utilized, for example, to effect the simple connect/disconnect operations and to effect other control functions within the array of geophones. These functions control the gathering of data in the field to fit the stacking plans and other requirements of the particular field experiment.
Field control of data gathering in accordance with stacking plans and other field techniques and requirements are important concepts in seismic exploration. These requirements demand flexibility in field operations and require data gathering instruments which can continuously control the parameters required to set-up the various field experiments.
In "Cable Systems", control is exercised by transmitting signals along these long and complex cables to effect the desired control functions within the data gathering system. Selected geophones and/or recorders are activated to detect and store the seismic data. Control signals transmitted through these cables control the simple connect-disconnect operations for single geophones or entire groups of geophones, usually prior to the recording period, in the process of expediting spread set-up for common-depth-point recording. Control signals may also be employed to connect individual geophones within each group to change the array geometry as a function of time. Control signals are further utilized to change individual seismometer amplitude settings and phase settings; this permits time variable beam steering as well as adjusting the frequency response of entire groups or individual geophones. Control of seismometer amplitudes settings and control of phase settings are functions useful in attenuating noise. By changing the dimensions of geophone arrays as well as amplitude and phase responses during the recording period, reflection-signal- to-noise ratios can be maximized.
It is now desirable to extend the control of data gathering operations to cableless distributed control systems. Advances in the area of seismic data gathering instrumentation have made it possible to distribute portions of the data processing and data recording functions to wireless ("isolated") recording stations ("isolated distributed recorders"). These "Isolated Distributed Recording Systems" eliminate the need for complex cable interconnections and provide greater flexibility in field operations. Isolated Distributed Recording Systems are particularly useful in areas of rugged or mountainous terrain, where the use of cable systems is greatly restricted. In place of the interconnecting cable system, which provides the communication/control link on a distributed cable system, Isolated Distributed Recording Systems utilize a Radio-Telemetry System or a Time Synchronization System.
Control of field data gathering operations in current Isolated Distributed Recording Systems may briefly be described as follows:
Radio-Telemetry Systems utilize coded radio commands to selectively activate isolated distributed recorders. Coded radio commands are also employed to control the frequency response of individual geophones or entire geophone groups. U.S. Pat. No. 3,916,371 to Broding is an example of a geophone control system which may be employed in a Radio-Telemetry System as well as in a Distributed Cable System.
Radio-Telemetry Systems, however, suffer from difficulties in reliably transmitting and receiving the radio frequencies that are usually allocated to this type of service. To a large extent, this has resulted from the difficulties inherent in separating the radio waves emanating from the individual transmitting devices and separately amplifying them without incurring serious "cross-talk" or distortion. Cross-talk occurs when a portion of one signal mixes with or overlays another signal. This difficulty is exacerbated by the fact that the individual distributed recorders or groups of recorders in a seismic plan are usually addressed by different encoded radio commands. Also, radio-controlled systems may use radio frequencies that are effective only along line-of-sight; such systems are often not effective in mountainous or obstructed terrains. Finally, in some areas, it is difficult or impossible to obtain permits for radio transmissions. Without these permits Radio-Telemetry Systems cannot be employed.
Time Synchronization Systems utilize an accurate time reference and a preselected time recording logic to operate the isolated distributed recorders independently of the central control station. Representative Time Synchronization Systems are disclosed in U.S. Pat. Nos. 4,281,403 to Siems, et al; 3,972,019 to Bassett; and 3,733,584 to Pelton, et al.
The preselected time recording logic eliminates radio transmission of control signals between each isolated unit and the central control station. Recording of seismic data at each isolated distributed recorder is either continuous (usually for one day, while a seismic survey is being "shot") or in accordance with a preselected time logic which effects recording at selected time periods regardless of whether seismic signals are being generated. Operation of geophone arrays is preselected and may not be altered except by changing the preselected conditions at each individual isolated distributed recorder.
A disadvantage of Time Synchronization Systems lies in the separation of the recording function and the seismic signal transmitting function. These functions are independent of each other. In a Time Synchronization System the recording periods, geophone array, amplitude response, frequency response, filter settings and other experimental variables are fixed in accordance with the preselected settings effected at each isolated distributed recorder. Flexibility of field operations is limited to the preselected settings. Control functions such as frequency control, array geometry control and/or individual seismometer amplitude and phase control is limited to a preselected time synchronization plan. A change in any variable would require adjustments to the individual isolated distributed recorders. This is an impractical, time consuming, and costly proposition, particularly when it is desirable to change field variables in accordance with stacking plan surveying and other noise reduction methods.
There exists, therefore, a need for a method and an apparatus for controlling the field functions of isolated distributed recorders in an Isolated Distributed Recording system which do not require complex cable interconnections, radio transmissions, nor dependency on a preselected recording time and function logic to control the recording of seismic data during seismic exploration activities.